1. Field of the Invention
The present invention relates to a thermal plate making apparatus for forming a stencil paper to be mimeographed of a stencil duplicator by scanning and heating a thermosensitive film by a line thermal head to make holes of dots in the film.
2. Description of the Related Art
A thermal plate making apparatus comprises a platen roller and a line thermal head between which a thermosensitive sheet of stencil paper passes through. The paper is selectively heated by heating elements of the thermal head which scans the paper in the horizontal direction while the paper is conveyed in the vertical direction so that holes are formed in the paper, whereby a stencil paper of a mimeographing machine is produced.
More precisely, in such a thermal plate making apparatus, the line thermal head comprises a plurality of rectangular heating elements disposed side by side in parallel to each other along a main scanning direction, i.e., horizontal scanning direction. Conductive wiring patterns are disposed in both longitudinal ends of each heating element along a sub-scanning direction, i.e., vertical direction electrically contacting with the heating element.
If resolution of the apparatus is 16 dpi, each heating element is formed in such a rectangular shape that the length in the vertical scanning direction is about 80 to 120 .mu.m and the length in the horizontal scanning direction is about 40 to 55 .mu.m, the elements being arranged at a pitch of 62.5 .mu.m along the horizontal scanning direction.
Such a line thermal head is also used as a head of a thermal printer and arranged in such a manner that the paper is fed at a pitch of 62.5 .mu.m in the vertical scanning direction, that is the same as the horizontal scanning direction. Therefore, if a stencil paper is scanned and heated by the line thermal head, two adjacent dots (holes) for passing through an ink become continuous because each boundary portion between the dots is destroyed. If papers are printed with the use of such a stencil, an overly large amount of ink passes through the continuous holes of the stencil so that the ink of the printed paper stains the back side of the preceding or subsequent printed paper when the printed papers are stacked one above the other.
In order to obviate such a problem, a thermal plate making apparatus is proposed in Japanese Patent Application Laying Open (KOKAI) No. 2-67133. In the proposed apparatus, the line thermal head comprises a plurality of heating elements, the length of each element in the vertical scanning direction being shorter than the pitch of the element arrangement in the horizontal scanning direction. For example, the length of the element is about 60 .mu.m along the vertical scanning direction and the elements are disposed at a pitch of 62.5 .mu.m in the horizontal scanning direction.
In accordance with such an arrangement, the thermal energy generated from each heating element is reduced, which enables the device to reliably form individual ink holes independently from the adjacent holes interposing a predetermined boundary space therebetween.
Therefore, the line thermal head mentioned above makes it possible to produce a stencil paper for a mimeographing machine in which the problem that the back side of the printed paper is stained by ink is attenuated.
In accordance with the above-mentioned thermal head structure, the rectangular heating element has four corners from which heat is effectively transferred and radiated to the outside thereof so that the temperature of the corners is reduced. Therefore, the ink hole formed by the heating element is rounded rather than rectangular.
Accordingly, a large blank space is formed at a center portion surrounded by four rounded ink holes in the stencil so that when the stencil is used for printing a paper, a white blank is formed in a portion where the ink is to be printed all black.
Besides, a problem arises in connection with the temperature distribution in the heating element of the line thermal head mentioned above as described hereinafter.
With respect to the long element having a length 85 to 120 .mu.m in the vertical scanning direction, the temperature changing range is small, that is, the temperature of the element does not change much in the longitudinal direction thereof and the peak temperature is relatively low. On the other hand, with respect to the short element having a length about 60 .mu.m in the vertical scanning direction, the temperature changing range is large, that is, the temperature of the element changes remarkably in the vertical scanning direction and the peak temperature is high. Therefore, when the temperature of the element is fluctuated, since the temperature changing range of the short element is wide in comparison to the long element, it becomes difficult to evenly form ink holes in the stencil paper with the use of the short elements which are used in the line thermal head to obviate the problem of staining the printed paper by the ink.
Besides, the durability of each element is dependent on the peak temperature of the element, therefore, the service life of the short element is shortened. That is, as the length of the element in the vertical scanning direction is shortened, the unevenness of the resistance of the element becomes large so that the temperature changing range becomes large. Accordingly, the peak temperature becomes large, which shortens the life of the apparatus.